I have the opposite problem: I recognize regular soda by its terrible aftertaste. Drinks sweetened with aspartame don’t leave an aftertaste in my mouth.

The rarest of the rares

I dislike plain aspartame, but most soft drinks here (Ireland, this stuff is weirdly regional) now seem to use it in combination with acesulfame K; this mixture is much less objectionable (apparently acesulfame K on its own is also fairly unpleasant; they mask each others' aftertaste).